1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for controlling a subsea tree with controls provided on a blowout preventer stack.
2. Discussion of the Background
During the past years, with the increase in price of fossil fuels, the interest in developing new production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel.
Conventionally, wells in oil and gas fields are built up by establishing a wellhead housing, and with a drilling blowout preventer (BOP) stack installed on top of the wellhead, drilling down to produce the well hole while successively installing casing strings. When the drilling is finished, the well needs to be converted for production. For converting the cased well for production, a tubing string is run in through the BOP and a hanger at its upper end landed in the wellhead. Thereafter the drilling BOP stack is removed and replaced by a Christmas tree having one or more production bores containing actuated valves and extending vertically to respective lateral production fluid outlet ports in the wall of the Christmas tree.
This arrangement has involved problems which have, previously, been accepted as inevitable. Thus, some operations down hole have been limited to tooling which can pass through the production bore unless the Christmas tree is first removed and replaced by a BOP stack. However, this involves setting plugs or valves, which may be unreliable. The well is in a vulnerable condition whilst the Christmas tree and BOP stack are being exchanged and neither one is in position, which is a lengthy operation. Also, if it is necessary to pull the completion, consisting essentially of the tubing string on its hanger, the Christmas tree must first be removed and replaced by a BOP stack. This usually involves plugging and/or killing the well.
Another difficulty that exists in the subsea wells, relates to providing the proper angular alignment between the various functions, such as fluid flow bores, and electrical and hydraulic lines, when the wellhead equipment, including the tubing hanger, Christmas tree, BOP stack and emergency disconnect devices are stacked up. Because there are many different designs and manufacturers for trees and BOPs, ensuring proper alignment of the functions cannot practically be achieved.
FIG. 1 (which corresponds to FIG. 2A of U.S. Patent Application Publication no. US 2010/0025044 A1, the entire content of which is incorporated herein by reference) shows a conventional BOP stack 10 provided on top of a wellhead 12. A subsea tree 14 is provided between the stack 10 and the wellhead 12. Subsea tree 14 has a port 15 for receiving hydraulic and other signals. The wellhead 12 is attached to the ocean floor 16. Various rams 10a-e are provided in the stack 10 for sealing the well when necessary. A connector 18 is configured to connect the stack 10 to the tree 14. The configuration illustrated in FIG. 1 may be used when work need to be performed inside the well. It is noted that in this configuration no control is provided to tree 14 as the port 15 is not connected to any control system. Also, it is noted that currently the BOPs are not functionally connected to the tree.
As discussed above, when the well is in production, the BOP stack 10 is removed. However, if further work needs to be performed on the well, the BOP stack 10 has to be brought back, which makes the production well not operational for an extended amount of time.
An alternative to using the BOP stack for doing workover is the usage of an Installation WorkOver Control System (IWOC) which is illustrated in FIG. 2 (which corresponds to FIG. 2B of U.S. Patent Application Publication no. US 2010/0025044 A1). FIG. 2B shows the IWOC 19 including an electrical-hydraulic control of tree functions, lower marine riser package (LMRP) 20, emergency disconnect package (EDP) 22, etc. The IWOC is controlled by an IWOC umbilical 26 that communicates with a vessel or rig at the surface. Hydraulic lines 28 and 30 communicate with the IWOC umbilical 26 and provide hydraulic pressure to the tree 14 (via port 15) and to a hydraulic control unit 32. The IWOC umbilical 26 also provides electrical communication to a port 34.
However, for using the IWOC alternative, the operator of the well needs either to rent the IWOC equipment (which today costs in the millions of dollars range) or to own the IWOC equipment (which today costs in the tens of millions of dollars range). These high costs associated with the IWOC equipment are undesirable for the operator of the well. Additionally, many times the IWOC system must be integrated into a BOP systems's LMRP, which entails a great deal of modifications to the BOP when installing and removing. These operations add considerable expense for the operator. Accordingly, it would be desirable to provide systems and methods that are better than the background art.